The invention relates generally to gas turbine engines and more particularly to a system and method for re-tasking, redesigning and/or manipulating the use of coolant injection holes to enhance the mixing of a gas turbine blade or vane wake to reduce HPT/LPT (high pressure turbine/low pressure turbine) interaction losses and improve the thermal durability of the turbine.
Turbine blade/vane wakes interact with turbine blade/vane shock wave systems and, if not fully mixed, with downstream blades/vanes. This process generates higher losses, and often requires extra axial spacing between the HPT and LPT to fully mix the wakes. Besides the aero-performance benefit of wake mixing, its benefit can also be extended to include the thermal effect. The wakes coming off of the film-cooled turbine vanes and blades have temperature deficits in addition to the pressure deficit. The low-momentum fluid in the wake of an airfoil (vane or blade) migrates downstream, and is ingested and turned by the downstream airfoil. The left side of FIG. 13 illustrates the wake migration. The right side of FIG. 13 illustrates that turning of the fluid in the downstream blade row causes the low-momentum fluid to accumulate near the suction side of the airfoil, while the higher momentum fluid outside of the wake moves toward the pressure side of the adjacent airfoil. Since the wake fluid is cooler compared to the fluid outside of the wake, the thermal segregation occurs in the passage of the downstream airfoil row where the coolant in the incoming wake cannot reach the pressure side. As a result, the pressure side of the downstream airfoil is hotter than the adjacent suction side, which may cause overheating of the pressure side.
Further, the wake of an upstream airfoil also introduces a secondary flow, which is often referred to as “slip” velocity, or “negative” jet if the wake has a lower velocity than surrounding flow, or “positive” jet if the wake has a higher velocity than surrounding flow. A velocity triangle illustrated in FIG. 14 depicts the negative and positive jets. Kerrebrock, J. L. and Mikolajczak, A. A., 1970, “Intra-Stator Transport of Rotor Wakes and Its Effect on Compressor Performance”, ASME J. Eng. Power, Vol 92, pp. 359-370, describes that even if the fluid temperature is the same everywhere, as the flow migrates to the downstream, this secondary flow causes the transport of the fluid across the airfoil passage, which in turn causes a temperature gradient across the passage.
FIG. 1 is a perspective diagram depicting placement and operation of fluidic injection elements 10 near to but not in the trailing edge 12 of a turbine blade 14 that is known in the prior art to enhance mixing of a wake following the trailing edge 12 of the turbine blade 14. At least one fluidic-generated vortex 16 operates to enhance mixing of a wake 22 following the trailing edge 12 of the turbine blade 14. The fluidic-generated vortex 16 is generated via one or more fluidic injection elements 24 integrated near the trailing edge 12 of the turbine blade 14. Each fluidic injection element 24 is configured to inject a fluid 26 such as a stream of air into a trailing edge region of the turbine blade 14 to enhance mixing out of the wakes behind the trailing edge 12 of the turbine blade 14. The enhanced mixing out of the wakes results in a wider wake 22.
FIG. 2 illustrates a typical turbine blade 30 having an rounded style trailing edge cooling scheme that is known in the prior art, viewed in a top-down perspective in which shaded areas 32 are metal and hollow areas 34 are coolant flow pathways. Turbine blade 30 comprises a trailing edge slot 36 with straight inner walls. FIG. 3 illustrates a bleed slot trailing edge cooling scheme having a thinner trailing edge comprising a trailing edge slot with straight inner walls 38.
Although fluidic injection elements/cooling holes and slots have been employed in the prior art to provide both aerodynamic performance benefits and cooling of turbine blades/vanes, such techniques have not yet successfully addressed aerodynamic losses resulting from wake and shock interactions, both steady and unsteady for transonic vanes/blades as well as unsteady losses due to wake interactions when blades/vanes are subsonic. Further, high pressure turbines where the vanes are heavily cooled to ensure durability in high temperature environments, e.g. film and trailing edge cooling, can result in unsteady thermal wake segregation effects that lead to the unexpected heat-up of endwalls, e.g. platforms and blade/vane tips, of downstream blades/vanes. Large temperature gradients are caused by the cold wakes in combination with hot post-combustion gases, which leads to a thermal wake migration effect.
In view of the foregoing, it would be advantageous to provide a system and method to reduce both the total pressure gradient and the total temperature gradient. The system and method should re-condition the flow that goes into the downstream airfoil row via mixing of the thermal wakes of an upstream blade row to reduce the thermal load of the adjacent downstream blade row by reducing the thermal segregation effect within the downstream blade row. FIG. 16 illustrates the mixing of velocity wakes and thermal wakes in a single sketch. Two parameters are introduced to measure the wake mixing. The first is Wake Velocity Ratio (WVR), which is defined herein as a ratio of velocity integral in the wake region and that in the free-flow region. The other parameter is Wake Temperature Ratio (WTR), which is defined as a ratio of mass-weighed temperature integral in the wake region and that in the free-flow region.